The present invention relates to controlling access to a shared document in a communication network of the “station-to-station” or distributed type, commonly referred to as having “peer to peer” topology.
Over the last few years, station-to-station networks have become an alternative to client/server networks, widespread until now. This is because, through their distributed architecture, station-to-station networks make it possible to share a large number of digital data items between a large number of users, without also requiring an expensive infrastructure.
In practice, in a station-to-station network, each station acts as client and server. Thus, each station can request a digital data item or document from any other station whatsoever on the network and the data exchange can take place directly from one station to another.
Subsequently, the term “digital document or data item” applies to both digital images or videos, or also to digital texts or the like.
Thus, in a station-to-station file exchange, each station can be both client and server.
This means that the digital data received by a client can be supplied to other users by the server of this client.
Digital data accessed by many people can therefore be replicated on a number of machines and supplied by more servers.
The system therefore adapts on its own to demand and the communication and storage costs are distributed between all the servers.
On the contrary, in a conventional client/server system, the data are supplied by a single server or by a set of machines fixed in advance.
The capacity of these conventional servers must be proportioned in advance which leads either to over-proportioning (the cost of the server is then too high), or to under-proportioning (the data are not supplied sufficiently quickly).
Another advantage of the station-to-station system is that the digital data are supplied directly from the machines of the users.
The storage space can therefore be considered in practice as unlimited.
However, station-to-station networks are unstable. This is because client devices (and consequently server devices) periodically connect to or disconnect from the network, thus making the presence of data highly uncertain. Moreover, the addresses of the client and/or server devices are unpredictable and liable to be different at each connection.
The result of this is that accessing the contents in a station-to-station type communication network also constitutes a great difficulty since the latency time for obtaining the data item is no longer due simply to the time necessary for retrieving the data as in the conventional client/server topology, but also the time for searching for a server device having this data item available. Depending on the topology of the station-to-station network concerned, this search phase can be non-negligible.
In order to remedy these drawbacks, a known solution consists of using a central server that guarantees a minimum quality of service. Such a solution is referred to as using Hybrid station-to-station networks.
In a known manner, sharing a digital data item through a hybrid station-to-station communication network consists of the sequence of the following steps:                selecting the document to be shared;        associating a unique identifier making it possible to find the location of the document in the network;        calculating a thumbnail image from the original data item;        updating an index table at the central server by associating an identifier of the shared document with one (or more) station(s), each of these stations being a server for this document.        
This series of known steps is used in the majority of station-to-station networks for sharing a document with a community of users, that is to say that anyone can access this shared document from the moment this person knows the identifier. This is referred to as public sharing.
On the other hand, a document can be shared with a group of friends and the accessing of this document restricted to this group of friends. This is referred to as restricted access sharing (private sharing).
In this case, the private sharing of a document requires two additional steps:                selecting the addressees; and        sending notification to each of these addressees so as to inform them that a new document is capable of being shared.        
In practice, the present invention finds a preferred, but non-limiting, application in a hybrid distributed communication network with restricted sharing.
Unlike conventional client/server systems, the shared data item in a station-to-station network is available on a number of servers, which increases the risk of malevolent operations with regard to the access to this data item thus shared.
It is therefore necessary to put in place an access control adapted to the sharing of this data item on each server.
In practice, each server is a machine belonging to a user who must be able to totally control what takes place on his machine. For example, a user must be able to verify what is stored locally. He must also be able to restrict the access to a data item on his machine or alternatively share it with other users.
The putting in place of such an access control mechanism restricts the number of servers who agree to supply a data item to a user, which is contrary to the main advantage of the station-to-station system where a data item in great demand can be supplied by many servers.
The Applicant was faced with the problem of providing an access control adapted to the sharing of a document in a station-to-station network which does not reduce too greatly the number of servers available for a user while being relatively efficient, not very expensive and simple to put in place.